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RE: CO2 and BOD



Lin,

I doubt the CO2 forms carbolic acid, that's the stuff derived from coal tar
and is very toxic. You need to check with the dye producer since this sounds
like you are dealing with the dyeing process.  Your basic choice for acids
are mineral (HCl, sulfuric, nitric) or organic (citric, acetic).  All of the
organic acids will increase BOD.  All of the mineral acids are hazardous.
Citric acid can also cause problems with you need to remove metals prior to
discharge.

CO2 is finding more and more use as a safe and effective way of lowering pH.
However, the effect tends to be temporary since the CO2 would prefer to be
in the gas phase.  I'm sure many more reactions enter into the picture.

Of the three common mineral acids, I would guess sulfuric acid to be
favored.  Both HCl and nitric can generate fumes that pose a risk.  The main
problem with sulfuric is that it will increase TDS levels in the discharge.
Its effect on the dyeing chemistry is unknown.

Since citric tends to be working, I would favor improvement to the existing
operation. You didn't state why they are feeding in granular acid but they
are definitely asking for problems. Dry powders should be dissolved or
slurried and then fed into the tank. This allows much better pH control and
regulation. Good luck.

Mike.callahan@jacobs.com

-----Original Message-----
From: Hill, Lin [mailto:lhill@des.state.nh.us]
Sent: Thursday, April 27, 2000 12:37 PM
To: 'p2tech@great-lakes.net '
Subject: CO2 and BOD


Dear P2Techies:

A client makes high quality woolen goods.  They are permitted for, and use,
CO2 for neutralization (to lower the pH) in their onsite treatment plant.
The CO2 forms carbolic/carbonic acid which lowers the pH.  The pH has to be
kept low in the process tanks in order to keep the colors in the fabric.  As
a buffer, and for final pH adjustment, they add granular citric acid to the
final contact tank.  The problem is, the citric acid dramatically increased
the BOD level and Packard is charged for sewer use based on the pounds of
BOD they're discharging.  It's expensive.

Apparently, it's extremely difficult, even with monitoring probes, to match
the citric acid volume to the exact pH in order to reduce the citric acid
(and BOD) loading of the discharge.  There's a pH alarm on the final tank
and when it goes off, the workmen quickly shovel the citric acid into the
tank.  They're using 6,000 to 10,000 lbs of the stuff per year.

Good P2 calls for better process monitoring/control, either mechanical or
human as that's the real cause of the problem.  She said she'd consider a
visit from our office, but in the meantime,  could we research another
chemistry to help them adjust their pH.  The citric acid was already a P2
strategy since it was purchased to replace a RCRA hazardous acid.  Is there
another non-hazardous acid treatment which wouldn't result in the high BOD
problem?

Thanks.
Lin K Hill
NHPPP
603-271-2902